1. Field of the Invention
The present invention relates to a toner used in a developer in developing an electrostatic charge image formed by an electrophotographic method, an electrostatic recording method, an electrostatic printing method, etc., an image forming method, and an image forming apparatus.
2. Description of the Related Art
An electrophotographic method generally includes the steps of forming a latent electrostatic image on a photoconductor that utilizes a photoconductive material (hereinafter otherwise referred to as “latent electrostatic image bearing member”, “image bearing member” or “electrophotographic photoconductor”) with the use of various units; developing the latent electrostatic image with toner so as to form a toner image; transferring the toner image onto a recording medium such as paper; fixing the toner image, which has been transferred onto the recording medium, on the recording medium by heating, pressure, thermal pressure, solvent vapor, etc.; and cleaning the photoconductor by removing toner that remains thereon.
A developer used in electrophotography, electrostatic recording, electrostatic printing, etc. is firstly supplied for development on an image bearing member, such as a latent electrostatic image bearing member on which an electrostatic charge image is formed, in a developing step, then the developer is transferred from the image bearing member onto a transfer medium such as transfer paper in a transfer step and subsequently fixed on the transfer medium in a fixing step. As developers used for developing electrostatic charge images formed on latent image bearing surfaces, two-component developers, each of which is composed of a carrier and a toner, and one-component developers (magnetic toners and nonmagnetic toners), which do not require carriers, are known.
Conventionally, so-called pulverized toners, which are each produced by melting and kneading a binder resin such as a styrene resin or polyester resin together with internal additives such as a colorant and then finely pulverizing the melted and kneaded matter, have been widely employed as dry toners used in electrophotography, electrostatic recording, electrostatic printing, etc.
In a toner producing method based upon a pulverization method, in order to secure uniformity in the shape of particles of a toner obtained, it is important to uniformly disperse constituent materials of the toner and then pulverize them. Basically, since the shape of particles of a pulverized toner is indefinite, and cross-sectional shapes formed upon pulverization vary from particle to particle, it is difficult to control the shape and structure of the particles of the pulverized toner. Also, if internal additives such as a colorant, a releasing agent and a charge controlling agent are added in large amounts, pulverization at the interface between a binder resin and the internal additives in a pulverizing process easily causes the internal additives to be exposed on the surface, and variation in chargeability or the like arises among individual toner particles, thereby causing a problem in which toner properties such as fluidity and chargeability degrade.
In recent years, the demand for improvement in image quality has necessitated making toners smaller in particle diameter; however, as the toners are made smaller in particle diameter, the following problems arise. (1) The pulverization energy exponentially increases. (2) Degradation of fluidity, which is also caused by indefinite shapes of toner particles, becomes noticeable and the toner supplying ability, the transferability and the cleaning ability degrade.
(3) Variation in chargeability among individual toner particles, which is caused by pulverization at the interface between a binder resin and internal additives, becomes noticeable.
These days, toner producing methods based upon chemical methods (for example, suspension polymerization method, emulsion polymerization aggregation method, dissolution suspension method, polyester elongation method, phase inversion emulsification method, etc.) for producing toner in solvent are being examined.
The suspension polymerization method is a method for producing a toner by dispersing into a monomer a polymerization initiator and internal additives such as a colorant, a releasing agent and a charge controlling agent, suspending this dispersion solution in a dispersant-containing aqueous medium to form oil droplets, and then raising the temperature so as to subject the monomer in the droplets to a polymerization reaction (refer to Journal of the Imaging Society of Japan Vol. 43, No. 1, pp. 33-39 (2004)).
The following explains an example of the emulsion polymerization aggregation method. First, a colorant is dispersed in a surfactant aqueous solution. Meanwhile, a polymerization initiator, a styrene monomer and an acrylic monomer are added to the surfactant aqueous solution so as to produce a resin emulsion by emulsion polymerization. The colorant dispersion solution, the resin emulsion, and a dispersion solution containing suitably selected other internal additives such as a releasing agent and a charge controlling agent are mixed together, and associated and grown with the addition of a pH adjuster and an aggregating agent so as to have a desired particle diameter, then the mixture is heated and agitated to fuse fine particles, thereby producing a toner (refer to Japanese Patent (JP-B) No. 3141783 and Journal of the Imaging Society of Japan Vol. 43, No. 1, pp. 40-47 (2004)).
The dissolution suspension method is a method that involves volume contraction, including a step of preparing a suspension in which an oily component formed by dissolving a binder resin in an organic solvent capable of dissolving the binder resin is suspended in an aqueous component, and a step of removing the organic solvent from the suspension. Internal additives such as a colorant, a releasing agent and a charge controlling agent are dispersed and dissolved, together with the binder resin, in a volatile solvent such as a low-boiling organic solvent, and this dispersion solution is suspended in a dispersant-containing aqueous medium to form oil droplets, then the volatile solvent is removed. As opposed to the suspension polymerization method and the emulsion polymerization aggregation method, this method is superior in that resins able to be used therein are versatile, notably in that polyester resins useful in a full-color process for which transparency and smoothness of an image portion after its fixation are required can be used (refer to Japanese Patent Application Laid-Open (JP-A) No. 07-152202 and Journal of the Imaging Society of Japan Vol. 43, No. 1, pp. 48-53 (2004)).
The polyester elongation method includes a step of preparing a dispersion solution by emulsifying and aggregating in an aqueous component an oily component formed by dissolving a polyester resin that contains a reactive resin and serves as a binder resin in an organic solvent capable of dissolving the polyester resin, and a step of performing a polyester elongation reaction while removing the organic solvent from the dispersion solution. As opposed to the suspension polymerization method and the emulsion polymerization aggregation method, this method is also superior in that polyester resins useful in a full-color process for which transparency and smoothness of an image portion after its fixation are required can be used, and makes it possible to control the viscoelasticity of a toner by means of the elongation reaction and thus fix an image in a wide temperature range (refer to Journal of the Imaging Society of Japan Vol. 43, No. 1, pp. 54-59 (2004)).
The phase inversion emulsification method includes dispersing and dissolving a binder resin and internal additives such as a colorant, a releasing agent and a charge controlling agent in a volatile solvent such as a low-boiling organic solvent, injecting a continuous aqueous phase into this dispersion solution so as to change the phase from a W/O dispersion system to an O/W dispersion system and thereby to form oil droplets, and subsequently removing the volatile solvent. This method is also superior in that resins able to be used therein are versatile, notably in that polyester resins useful in a full-color process for which transparency and smoothness of an image portion after its fixation are required can be used (refer to JP-B No. 3063269 and JP-A No. 08-211655).
Toners capable of effectively exhibiting desired functions in view of recent environmental problems, for example capsule toners and core-shell toners, are known to be among toners produced in accordance with such chemical methods.
Compared with pulverization methods, those chemical methods make it possible to produce toners which are small in particle diameter and narrow in particle size distribution.
In order to prevent degradation of fluidity, transferability and cleaning ability owing to reduction in the size of toner particles, which arises in pulverization methods, and to prevent decrease in chargeability and degradation of temporal stability and environmental adaptability owing to the surface hydrophilicity of toner, which arise in the case of chemical toners, conventional toners are generally formed by attaching inorganic or organic fine particles onto surfaces of toner particles such that the adhesion of the toners is reduced by the effects of these fine particles. Additionally, also in order to provide sufficient fluidity in conveying toner from a toner container to a developing unit, inorganic or organic fine particles are generally attached onto surfaces of toner particles.
For those fine particles, hydrophobic fine powders typified by hydrophobic silica, etc. (refer to JP-A No. 52-30437), fine silica particles mixed with fine aluminum oxide particles, fine titanium oxide particles, etc. (refer to JP-A No. 60-238847), alumina-coated fine titania particles (refer to JP-A No. 57-79961) and so forth are known to be used. As for titanium oxide, the following have been proposed: titanium oxide having an anatase crystalline structure (refer to JP-A No. 60-112052), aluminum oxide-coated titanium oxide (refer to JP-A No. 57-79961), and fine titanium oxide particles surface-treated with a coupling agent (refer to JP-A No. 04-40467). In general, however, silica, which yields the greatest fluidity-providing effect, is used. By using these hydrophobic fine powders such as silica, the fluidity, the developing ability and the transferability can be improved to a fairly great extent.
However, when used in a copier, a printer, etc., these external additives held on surfaces of toner particles are always subject to mechanical stress in a developing device, a transfer unit, a cleaning unit, etc.; thus, as the external additives are embedded in the toner particles or detach from the surfaces thereof, the adhesiveness of toner increases with time, which leads to a decrease in transfer efficiency and cleaning reliability.
In recent years, it has been required that images with a quality comparable with that of offset printed images or photographs be realized by using electrophotographic dry toner, and it has been hoped that the size of toner particles can be reduced to obtain a high resolution, that the amount of toner attached can be reduced and the pile height of a toner layer can be lowered to obtain a texture which is as natural as that of offset printed matter, and that the transparency of color material can be further enhanced to widen the color reproducible range.
Generally, the amount of pigment contained in a toner is increased in order to reduce the amount of toner attached, lower the pile height of a toner layer and maintain an image density; however, when the amount of pigment is increased, image fixation may be hindered, and the presence of the pigment on the surface may possibly make charging unstable and thus cause image degradation. As to chemical toners produced by chemical methods such as suspension polymerization method and dissolution suspension method, an increase in the viscosity of solution makes it difficult to form droplets and obtain particles in some cases.
As means for further enhancing the transparency of color material so as to widen the color reproducible range, fine dispersion of pigment and use of dye are known.
As to techniques of finely dispersing pigment, especially in order to stabilize the dispersion of the pigment in organic solvent, a graft polymer pigment dispersant has been proposed as described in JP-A No. 2005-232443, and a pigment dispersant using a silicone macromer has been proposed as described in JP-A No. 2005-36220. JP-A Nos. 2007-94352, 2007-94351 and 2007-155926 each describe an appropriate means of dispersing pigment in a method for obtaining toner particles by forming a toner composition liquid into oil droplets in an aqueous medium and solidifying the oil droplets into solid particles. When pigment is finely dispersed to a greater extent, a larger amount of a pigment dispersant is required to stabilize the dispersion, and thus there are such problems that the charge stabilization of toner is hindered and the fixation properties of the toner are greatly changed. As techniques for finely pulverizing pigment, use of a ball mill and use of a bead mill are generally known. In recent years, for finer pulverization, pulverization methods utilizing laser abrasion have been proposed as described in JP-A Nos. 2004-267918 and 2005-238342, methods for finely dispersing pigment by means of dissolution and deposition have been proposed as described in JP-A Nos. 2004-331946, 2004-091560 and 2006-193681, and methods for producing pigment in the form of fine particles by spraying and drying pigment solution have been proposed as described in JP-A Nos. 2005-518278 and 2006-152103; however, in each method, use of a large amount of a pigment dispersant is required to stabilize the dispersion, and thus there still remain such problems that the charge stabilization of toner is hindered and the fixation properties of the toner are greatly changed.
Although superior in color tone and transparency, dyes present such problems that they have poor light resistance, they move when stored, causing image bleeding, and they stain a film or the like when kept in contact with the film or the like. Solutions to these problems include use of a polymeric dye. JP-A No. 62-245268 describes a polymeric dye in which a bisphenol dye is introduced into a polyester skeleton, JP-A No. 63-85644 describes a polymeric dye produced by polymerizing a vinyl group-containing azo dye, and JP-A Nos. 01-147472, 01-147476, 01-161362, 01-161363, 01-161364, 01-161365, 01-164956, 01-164957, 01-164958, 01-164959, 01-173056, 01-173057, 01-173058, 01-173059, 01-173060, 01-173064, 01-173065, 01-173066, 01-173067, 01-173068 and 02-2575 each describe a polymeric dye to which a rhodamine dye is added, etc.; however, since special dyes are used therefor, there is such a problem that the polymeric dyes obtained are rather expensive.
Additionally, JP-B 3068654 describes an electrostatic photographic toner which contains a colorant obtained by reacting together a basic dye and a resin having a carboxyl group or sulfonyl group as a side chain group, and JP-A No. 2007-101708 describes colored particles and a color toner in which the amount of resin functional groups and the amount of dye reacting with resin are determined; however, the foregoing are similar to those obtained by a conventional technique in terms of particle size distribution and toner properties.
Thus, as to a toner which enables a wide color reproducible range and an image having a sharp color tone and high transparency, which has a sharp particle size distribution and favorable toner properties such as chargeability, environmental adaptability and stability over time, which does not produce waste liquid, contains no residual monomer and does not require a drying process and which is low in cost, and an image forming method using the toner, those capable of exhibiting sufficiently satisfactory performance have not yet been provided in reality.